JPS5911045A - Optical transmitting method and device - Google Patents

Abstract

Stimulated Brillouin Scattering, which occurs when light of narrow linewidth and above a threshold power is launched into a low loss optical fiber, is suppressed by time-varying the phase angle of the transmitted light waves.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to the transmission of optical signals. In particular, the present invention relates to a method and apparatus for transmitting light along a dielectric waveguide, a method and apparatus for digital optical communication transmitted over an optical fiber, and a method and apparatus for transmitting light that suppresses stimulated Brillouin scattering.

"Optics", "light" and related terms are herein defined broadly with respect to electromagnetic waves in the broader spectral range of visible light wavelengths.

[Background of the invention]

In optical communication systems, light modulated according to communication information is transmitted along a dielectric waveguide (more narrowly, an optical fiber). Currently, most optical communication systems, of which optical telecommunication systems are a notable example, employ a combination of non-coherent light and direct intensity modulation to convey digital information.

Considerable benefits are contemplated in terms of bandwidth utilization, transmission bandwidth, selection of appropriate modulation techniques, receiver sensitivity, etc. by using cohesin 1-light for transmission.

Non-coherent for transmission] - In different optical communication systems that use light and systems that use coherent light (hereinafter referred to as "cohesive systems"), narrow optical ray width (meaning that the wavelength spectrum is narrow) ) light sources must be used, and low-loss single-mode optical fibers are generally used as dielectric optical waveguides, especially in long-distance communications.

For example, light with a narrow beam width emitted from a laser light source
When it enters an optical fiber, especially an optical fiber with a low loss of 11, the output is higher than the dark value, and stimulated Brillouin scattering (St
imulated Br1llouin Scatte
ring is hereinafter referred to as rSBSJ. ) occurs within the optical fiber. This is described in 1, for example, the following document.

RG Sm1th, ”0ptical Power
llandling Capacity of Low
Loss 0ptical fibers as D
Eterminated by Stimulated Ra
man and Br1llouin Scatter
ing”.

8ppl Opt, 1972. II,
Pages 2489-2494; EP Ippen an
d RII 5tolen, ” Stimulat
ed Br1-11ouin Scattering
in 0ptical fibers”
+ 8pplPbys I, ett, Vol 2
L No. 11. l I)ec 1972;” 0p
tical Fiber Telecommunica
tions”, 1979゜8 academic Pr.
ess, New York (US) +
ed S E Milleret al, C
hapter 5” Non Linear Prop
erties of Optical Fibers”,
Pages 125-150, Section 5.3; P Labudde
et al+” Transmission of
NarrowBand lligh Power La
5er Radiation Through+ 0p
ti-cal Fibers″+ 0ptics Co
mmunications, Vol 32°No 3
．． Mar 1980.385390 pages; N Uesu
gi et al, ” Maximum Sing
le FrequencyInput Power i
n a Lor+gOptical Fiber De
Terminated by Stimulated Br1
llouin Scattering'', Elec-
Lronics Letters, 2B May 1
981+ Vol 17+ No 11° According to these documents, SBS is a process of stimulated scattering that converts an advancing light wave into a light wave traveling in the opposite direction and also causes a frequency shift.

When light is emitted with a power above the aforementioned darkness value, the amount of scattering increases rapidly until the light propagating forward through the fiber is almost unaffected by the emitted light. In addition to reducing the propagating light, SBS also causes disadvantages such as shift phenomena to multiple frequencies, increased back-coupling light of the laser light, and physical ID scratches in the optical fiber when the output light is larger. Wake up 4゛.

It should be noted that SBS is of course not limited to coherent systems, although it is of great importance for ml coherent systems where the use of narrow beamwidth light sources is forced. Rather, SBS can occur whenever the beam width, emission power, optical waveguide characteristics, etc. meet appropriate conditions.

S B S is one of the nonlinear processes that can occur in optical waveguides, and is generally less important for wide beam widths than for narrow beam widths. However, since its dark value is still generally lower than that of other nonlinear processes, SBS has been considered to pose a serious limitation to optical transmission systems (see above, especially RGS Smith, P Labudde,
and N Uesugi). This limitation clearly limits the practical maximum power output, and limits the degree of freedom in selecting light with a large linewidth for coherent systems, as is already the case.

Not only do most of the above-mentioned cited documents not make many checks on single-input power, they also do not make any effort to overcome this limitation regarding the dark value of SBS.

For example, N. Unisugi et al. in 2003 describes the occurrence of SBS in long single-moat silica fibers with low power inputs of a few mW in the near-infrared range. However, their research has focused on SB in coherent systems.
While focusing on the importance of S, -ζ does not suggest any remedies for it.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical transmission method and apparatus in which optical transmission using a dielectric waveguide is not adversely affected by SBS.

Another object of the present invention is to provide a technique for effectively suppressing S133 in optical transmission through a dielectric waveguide.

A further object of the present invention is to provide an optical transmission method and apparatus that suppresses SBS in optical transmission using a dielectric waveguide.

The present invention further provides the ability to transmit high power optical signals to optical fibers and other dielectric waveguides,
The purpose is to increase the relay interval of the transmission line.

[Features of the invention]

A first feature of the present invention is a method for transmitting optical signals in a dielectric waveguide, comprising a method of manually generating high power light emitted from one or more narrow beamwidth light sources, the light being transmitted through a dielectric waveguide. To ensure that SBS occurring in the wave path is sufficiently suppressed,
It is said that the phase angle is configured to change with time.

Another feature of the invention is an apparatus for transmitting optical signals in a dielectric waveguide, comprising one or more narrow beamwidth light sources and an optical fiber into which the output light of the light sources is input. is characterized in that the phase angle of the light wave changes with time so that SBS occurring in the dielectric waveguide is sufficiently suppressed.

The terms "high power" and "narrow beamwidth" here refer to the guided waveguide through which the optical signal is transmitted, where the optical energy is large enough to cause SBS and the spectrum of optical wavelengths is narrow. It means that. This degree is difficult to express numerically and is a property that should be defined experimentally based on individual conditions.

The following observations were made as a basis for carrying out the experiment. It is common to narrow the linewidth for a given optical waveguide and wavelength, but the power reduction that makes the SBS clear (=
J follow. In addition to its power, the SBS is defined by the properties of the dielectric waveguide and the wavelength of operation. Therefore, long low-loss fibers are generally susceptible to SBS, and the SBS darkness value tends to decrease with increasing wavelength.

Trial-and-error experiments are also required to determine the temporal change in phase angle required to sufficiently suppress stimulated Brillouin scattering. In order to be able to design such an experiment, it will become clear that more special R/I tests will be required in the future, either directly or analytically.

Help for conducting experiments will be obtained by theoretical models. Fortunately, the most common type of optical waveguide is a single-mode optical fiber, which has one or more transmission loss minima for wavelength, and one or more narrow linewidth light sources. It can be adjusted to operate near the minimum transmission loss point. Presumably each light source will be tuned to operate at wavelengths longer than 1 ttm.

The present invention provides a combination of e.g. Applicable to fiber. Silica optical fibers often have absorption losses down to 0.5 dB/km or less at 1.3 μm, 1000 m, or both. High power light waves according to the invention of 10 mW or more can be conveniently input into the fiber.

Continuous fiber lengths of 10 degrees or more can be used efficiently.

Operation at long wavelengths of fibers with low absorption losses at such frequencies requires
an, Sol, 5tate and Electron
ic Device 1978+2+129-137
) The possible length of the relay section will be i11 greater than this length. The SBS darkness value for continuous radiation and the minimum power convenient in connection with the present invention will generally be smaller. For example, such fibers may be fluoride glass fibers or the like, which obviously operate at wavelengths of 31 trn or more.

Furthermore, the present invention provides a method of transmitting information.

That is, one or more narrow beamwidth light sources are configured to transmit a high power optical carrier into an optical fiber, the carrier is modulated by the information being transmitted, and the modulated carrier is a guided optical fiber. It is characterized by being configured so that its phase changes over time so as to suppress scattering.

The invention further comprises one or more narrow beamwidth light sources, modulation means and optical fibers, the light sources or means for delivering high power information modulated optical carriers into the optical fibers. It is characterized in that the phase of the carrier wave is changed over time so that stimulated Brillouin scattering is sufficiently suppressed.

The change in phase angle mentioned here includes a change in phase angle due to modulation.

Further, throughout this specification, the term "modulation" should be understood to include "keying" as well.

"Keying" is a special case of modulation and is widely used in digital information transmission. Fortunately, demodulation of such general transmission waves is performed coherently.

The present invention is particularly suitable for transmitting digital information by performing binary phase shift keying (PSK) of optical) M transmission using a high-speed pit-ray. In this case, efficient SBS suppression can be performed by phase shift keying.

The depth of the phase gap is (2n+1)π, where n is 0 or an integer. Alternatively, by setting the phase shift close to an odd multiple of π, SBS can be effectively suppressed.

The invention is likewise suitable for digital transmission by binary frequency transition modulation (FSK) of high speed pit-rays. In this case, by increasing the frequency shift,
Sufficient SBS suppression can be achieved. PSK and F
Two case implementation experiments of SK are described below.

These experiments show how (2n + 1
) The answer is whether SBS can be sufficiently suppressed by performing PSK close to π, and how the frequency shift can be increased. In these experiments, SBS was observed using optical fiber output light and reflected light.

The present invention has the following two features. In other words, when PitRay 1 is 100 Mbit/s or more, especially 1G
It is effective when the speed is higher than bit/s. Furthermore, it cannot be applied to extremely low bit ratios in practice. According to the study, the effective lower bit rate is l bit/
It is s.

In the two cases mentioned above, the modulation technique itself is based on changing the phase of the carrier wave, and 1'i information modulation will be used to sufficiently suppress SBS. However, this, of course, does not preclude the possibility of using (; J month information modulation or adding the biased Old West value to the SBS suppression of the present invention.

But if amplitude modulation (e.g. eight Mr'LITUIIE
SIIIFTKEYING, ASK) is used depending on the transmission information, the 1n signal modulation at that time generally has little effect in suppressing SBS. For example,
This is because in the ASK system, the SBS darkness value is much larger than the average lighting power or continuous wave light source power. A similar problem arises if it is desired to utilize phase or frequency modulation that for some reason cannot suppress SBS by itself.

Therefore, the present invention provides means for modulating the phase in addition to modulating the information modulation, for example between a laser light source and an optical fiber, using a periodically driven optical phase modulator, thereby providing sufficient SBS Provides a way to obtain restraint. This includes:
A sine wave, square wave, or any other modulated waveform can be used.

When a plurality of signal frequency components with different frequencies (wavelengths) are added to an optical signal or an optical carrier wave, the phase angle of the optical signal is equivalently rotated, and this can be used to carry out the present invention. be able to.

Initial experiments were conducted using a periodically changing phase modulator or a carrier wave with set characteristics, without modulation by an information signal, and the effectiveness was confirmed. These experiments help determine the input optical power, optical fiber, beam width, phase modulation parameters, or required frequency shift, etc., provided for SBS suppression. A further logical model was used to guide the experiment.

A carrier wave containing many different frequency components can be generated from a single light source. For example, when modulating a single laser operating in two slightly different longitudinal modes of wave land with an overall mode beating effect. In many cases it is appropriate to modulate the power supply. Alternatively, two or more single frequency lasers can be used simultaneously.

Information modulation of the carrier wave is effective whether after or before adding the frequency components, or even simultaneously. It is convenient to perform this final information modulation and frequency component addition at the same time. This can be achieved, for example, by amplitude modulating the power supply of a single light source.

Demodulation of a carrier wave modulated to include a plurality of different frequency components can be performed using a detector with a coherent detection bandwidth smaller than the beat frequency for one of the frequencies.

In this case, only half of the transmitted optical power is effective for data transmission, so the 'JJ rate is reduced by 3 dB.

However, compared to the older proposed coherent amplitude modulation method, this ASK system can use higher input power due to SBS suppression and has the ability to increase the mesophase spacing by 1 Iif.

Lull modulated by a periodically varying phase modulator
Transmission, or multiple frequency components (1 added li)
T! Another way to utilize a transmitted wave is to use it as an optical phase reference for the demodulation of another wave containing frequency or phase modulated information.

Some frequency modulation or phase modulation schemes require simultaneous transmission of components of the tllll transmission as a reference for one optical phase. The present invention has found that under certain conditions, such a carrier may significantly improve SBS.

Periodically changing phase-modulated light or light containing multiple frequency components can be used not only for communication but also for continuous light transmission.

(Hereinafter referred to as superimposed margins) [Explanation based on Examples] More details will be described below using Examples and drawings.

The experimental equipment for detecting SBS shown in Figure 1 is as follows:
It consists of a radar light source 1, a test fiber 4, and devices 7.8 and 9 for power or frequency monitoring. Laser light source 1 is Nd:YAG with a single wavelength of 1.319/I m.
It's a laser. The variable attenuator 2 attenuates the light input from the laser light source 1 into the fiber 4. A polarizing filter 5 and a thick plate 6 having a wavelength of V4 are interposed between the laser beam #il and the optical fiber 4.

Devices 8 and 9 are calibrated Ge photodiode power monitors that monitor the frequency spectrum of the light passing through the test fiber 4.
erot) is an interferometer.

The following research experiments were now conducted for SBS and its suppression, as shown in FIGS. 2-5.

[Experiment 1]

Continuous single frequency Nd”: 1.319μ of YAG (Inno-1~Aluminum Garnet) laser
m transitions were used. This laser has a single longitudinal moat and diffraction removal, and the output power in TEMoo Yokote mode is approximately 10
It produced 0 mW. The beamwidth of the radar output light is measured by 300 MIIz scanning confocal Fabry-Perot interference δ1 in the free spectral range, and the resolution of the instrument is 1.5 M.
smaller than H2. This is ten times narrower than the natural Brillouin linewidth ΔνB.

FIG. 1 shows an experimental apparatus for observing SBS of a low-loss silica fiber.

The output of the laser 1 is attenuated by a circular variable density filter and is focused and input into the case 1-fiber 4 using the objective lens 3 of the microscope. Optical power dissipation from the near and far ends of the fine\ is monitored with calibrated Ge photodiodes. A 7.5 GIIz free-spectrum I-Lamb range scanning confocal Fabry-Béro interferometer was used to record the frequency spectrum of the scattered light. A common conclusion from fiber experiments is that the fiber must be cut back within a few meters of the input loop in order to measure the optical power in the modes guided into it.

The linear polarizer 5 and the A-wavelength thick plate 6 are used for the purpose of separating the laser and the fiber. However, strong S
Under BS conditions, this device proved inadequate in isolating the laser from the backscattered signal. The cause is the disturbance of polarized light within phi. Nevertheless, in radar ■, the frequency of the reciprocal scattered light is Nd
: This is because it operated in a stable single longitudinal mode even under conditions where it was considered that the YAG gain curve was shifted sufficiently strongly.

Experiments were carried out on a 13.6 km long Ge 02 doped single moat silica phino. This Fine\ has a core diameter of 9 μm, a refractive index difference of 0.3% with the cladding, a cutoff wavelength of 1.21 pm, and a cutoff wavelength of 1.32 μm.
The loss is 0.41 dB/km. According to the computer solution of the introduced mode from the measured refractive index, the wavelength is 1.32
In μM, the introduced mode diffusion (Guided Mod
e DistribuLion) is Δ-4,7X 10
-” rrf.

FIG. 2 shows the output power and human noise at each end of the fiber. White circles are measurements of forward light observed at the far end, and black circles are measurements of backward light observed at the near end. At low input power, the only output power monitored in the reverse direction is the fresle reflection from the cut end of the fiber. However, when the input power exceeds 5 mW, the output power in the reverse direction increases rapidly and nonlinearly, and the conversion efficiency for the despread wave approaches 65%6. The power radiated from the far end of the fiber is linearly related to the input power, with a linear loss of 5.6 dB. However, at input powers greater than 6 mW, the output cover saw becomes nonlinear. For input power exceeding IOm-, the forward output power is saturated and has a maximum value of about 2m.

Figure 3 shows the Fabry-Perot spectrum of reciprocal scattered light. A small amount of radar light instead enters the interferometer and serves as a calibration marker. The spectral portion labeled "-8 (Stokes)" exists only when the fiber input beam T exceeds 51 darkness value.

1' L , 1 is the laser frequency. If it is as shown in FIG. 3, the laser and the backscattered signal have two interference orders (free spectral range 7.5 GIIz), and the Stokes shift in that case is 12.7±0.2611z. This agrees well with the calculated value of 13.1GIlz.

, 2 is obtained from 2 Van/λ, the constituent letters are shown later, and the acoustic velocity is 5.96×103 m/s for fused silica. The line width displayed in the image is limited by measurement accuracy.

The frequency spectrum of the light emitted from the far end of the fiber has a strong component 1- at the laser frequency and a weak component 1- at the Stokes frequency. The Stokes frequency component is probably due to reflection from the laser output reflector. Surprisingly, no anti-Stokes or high-level Stokes shift morphisms were seen in this experiment, only those due to the feedback from the Radhe device (P Labbudde eL
al, 0ptics Comm1980, 32.3
(See pages 85-390).

[Experiment 2] Using the apparatus of Experiment 1, an experiment was conducted using a 31.6 km long single-seven-single-optical fiber cable. Its linear 11 loss is 17.4d at a wavelength of 1.32μm.
It is B. The experimental results were similar to the 13.6 km fiber, and SBS was observed when the input was greater than 6 mW.

Instead of the physical length, the effective interference length Le is (1, 3)
The formula, i.e., 31.6 km of cabled fiber, 7.7 km is almost the same as that of 13.6 km of fiber. Other fiber parameters are similar, and therefore, from equation (1, 1), it can be considered that the dark power of SBS is almost approximate for both fibers.

[Experiment 3] The equipment was the same as in Experiments 1 and 2. However, the laser operates both of the above in sequence. In the first single frequency configuration, the laser had an output power of about 1001 in a single longitudinal mode with a linewidth of less than 1.6 MIIz, and a Fabry-Béro interference needle was used.

Second, the dual-frequency form, i.e. two adjacent longitudinal motes with 27°
0MlI2 separated. In this case, the laser is about 2
501 output, equally distributed over two lines, and the measurement resolution was measured to be below 20 MIIzlu on both lines. In both systems, the laser output is diffraction-free TEM
oo It was in horizontal mode.

The fiber used was a 31.6 km long cable regenerated-mode fiber with a Kaneda loss of 17.4 dB at a wavelength of 1.32 μm. The theoretical SBS darkness value obtained with this fiber was 6 mW with a single frequency laser.

FIG. 4 shows the output from both ends of the fiber as a function of input power when the radar operates at a single frequency. Similarly, the white circles are the measured values of the forward light observed at the far end, and the black circles are the observed values of the backward light observed at the near end. It was found that the nonlinear reflection and transmission characteristics of SBS are in good agreement with theory when the input power exceeds 61.

FIG. 5 shows similar measurements when the laser is operated with dual frequencies. Similarly, the white circles are the measured values of the forward light observed at the far end, and the black circles are the observed values of the backward light observed at the near end.

It is observed that the optical linearity remains unchanged up to an input power of 9Qnll in the forward and reverse directions. 90mW is here
This is the maximum possible power in the experiment of ``.It was observed that the dark value power increased by 1 to 12 dB. (
It is estimated that Equation 3.2) appears when the ζ power level exceeds 850 mW using a beat frequency of 270 MIIz. This indicates that the darkness value has increased by 21 dB.

The following review of the theoretical model of operation of the present invention is intended to provide some guidance for the trial and error experiment and implementation described above.

In addition, it is easy to understand that the theoretical model that has been carefully examined is based on some underlying simplifying assumptions, and therefore does not impose precise limitations on the characteristics of the invention. Will.

For example, RG Sm1th (see above) and -Kai
er and M Maier (“Stimula
ted Rayleigl++ Br1llouina
nd Raman 5pectroscopy”, L
a5er l1andbook Vo12+ ed,
F I Arrecchi and E
OScl+ulZ-Dubois.

North 1loland, 8msterdam
1972. Starting from the small-scale constant state theory tested by ζ, the maximum continuous wave laser power PL that can be input to the optical fiber is S.
Before B SM is detected, it is given by the following equation.

GLe' 21 --- (1,
1) Here, G is the SBS gain coefficient.

--(1,2) where n is the refractive index, ρ0 is the material density, Va is the acoustic velocity,
It is assumed that B12 is the elastic/optical coupling coefficient of the fiber material, 8 is the effective cross-sectional area of the introduced mode whose peak intensity is given by P L /A, and the laser linewidth is small compared to ΔνB. ΔνB is the linewidth of spontaneous Prillouin scattering at room temperature (Ilz, Pth IM), and the coefficient is I for polarized fibers and 2 for others (RIf 5tol
en, IEE J Quart, Elec
, 1979. QE-15, 1157-1160
). The actual 9JJ interference length Le is given by the following equation.

I-e -α-' (l exp(-tx)
) - (1, 3) where - is the absorption coefficient (m-1) and is the fiber length. For long fibers used in communications, α1 is the norm, and therefore 1. e-α
-1. Low loss fibers have long interference lengths and therefore low SBS values.

We inserted the bulk parameter of fused silica into the equation of Gore et al. (R, I Pressley (ed)
+” 1landbook of I, aserS'
CI+emical Ruhber Company
+C1eveland, 1971 and J
Sct+roeder et al, JA
mer.

Ceram, Soc, 1973.56, 51
0-514): n = 1.451゜ρo = 2.21×10'kgm-"Va-5,9
6X 10 m 5 1112 = 0.286 Spontaneous line width 61 days is 38.4M at wavelength 1.0μm
1lz and varies according to λ2 (1) llei
nmanet at, l”hys, Rev,, 1
979+ B19.6583-6592). We set Δνe=22MIIz at 1.32 μm.

Additionally, we inserted the following values into a special 13.6 km test fiber.

α=9,5 x 10' m -" (0,41d
B/km tM loss) 1. -7,6 km Δ-4,7
The S I (S FA value of P L # 5.6 mN' can be determined).

Experiment 1 above concerns this test fiber.

In order to analyze the transient scattering process, we use the optical electric field E with a weak time-varying complex Fourier amplitude and the density wave ρ as an axis.
− bonding equation was used.

aEs / 9 z = iK2 0'
EL -t α lEs/2(2,1) 'rJI'/'ar---iKt EL+'Es-1'
p'--'<2, 2) (These formulas are based on F Shimizu, CS Wan
g+ and NRloemher en, 'Tl+
eory o(5takes pulse 5hape
sin transient stimulated
Ramam scattering'+el+ys
f? ev A, 1970. 2. 60-72 for comparison e,) The laser field (subscript L) enters the fiber 2-0 and travels in one direction. Su(・-kusu world(
The subscript S) advances in the −2 direction and grows from spontaneous scattering. This can be expressed as the Stokes field being injected at 2""20. Here, 2°-3α-1, α is the light absorption coefficient (RG Sm1th
reference). The positive (negative) in the traveling coordinate τ−L±z/v means the Leede (Stokes) field, and ■ is the group velocity of light (assumed to be much faster than the acoustic velocity).
It is. “-1 is the lifetime of the acoustic phonon.

The spontaneous Prillouin scattered line 1lvii (nz, p circle) is /π. The coupling coefficient is π n pi2 here n-βλ/2π, β is the light propagation velocity, and B12 is the length
, elastic/optical coupling constant, λ is the optical wavelength, ρ0 is the average density, V
a is the acoustic velocity and ε0 is the free space permittivity (S,, I units).

Detectable SBS occurs at a laser power for which the following inequality holds (see R G Sm1tl+ mentioned above)6
Inl Es, (0, r) /Es (zo, τ
)12≧18−−−− (2,3) For input power that does not exceed this critical level, SBS
There is little wear and tear on the laser field due to This is probably true for other nonlinear processes as well.

We essentially believe that the laser field in the fiber is determined by the injection field and linear absorption.

EL (Z, τ )=EL (0, τ
) exp(-cyz/2)----,
(2,4) The above (2,1) and (2,2) are Rieman
), and the Stokes field Es (Z, τ), which is the following formula with 2=0, is obtained (Carm
anet al and Daree” Transie
nt effects stimulated l
light scattering'', Opt Quan
t Electr+1975, pp. 7.263-279).

Us(0,τ) −(K i K 2 Z e )λexp (−α2
/2), EL (sail τ) × exp[-r”(τ-
τ′)] Ego(0, τ′)ES (z, r”) (W-
W' )→Xlt (j (4 to +22e (W-W
'l') d r' here ZQ-(1-exp(-α
2)]/αW(τ) −5”l EL (0, τ”)
12dτ"-('2.5)W' indicates W(τ'), 1
1 is a modified Bessel function. Using the above equations (2, 5), it can be confirmed whether the threshold conditions (2, 3) are increased by some input field.

We can evaluate the above equations (2, 5) in the special case of optical communications. In other words, a function m(1) that takes “0” and “l”
Two major simplifications can be made regarding the laser field modulating into a binary data stream expressed as . The first is the following assumption.

This is the time averaging of the modulation function m(t).

This converges for sufficiently large Δt, and is so for Δt<r~1. Room temperature of fused silica 1.32μm (RGSm4
th already mentioned), r/'1'' = 22 MHz, and l
' decreases with λ-2. We believe that the above assumptions are valuable for ultra-low disparity optical channels where Haira speeds exceed 100 megapips per second. (m is 2 using a formula that returns to zero for the balance coat ° or some other special method.
It changes from ) The following assumption is α−'>') v l”−
'is. We assume that this is α for 0.5 dB/km in a low-loss silica fiber, e.g. 1.3,17111
-1"-, 8,7km and y+"-'=3. ．． 5
It seems preferable for m.

1 This simplification allows a solution of the integral equation (2, 5) obtained in electric terms.

In order to consider different modulation techniques such as ASK, PSK, and FSK, we decided to start separating the amplitude and phase fluctuations of the input field.

, 1 ““°” pa = “°a (tl exp (”
a(tl)---(2,6): . . 0.5. Yo constant, 3 Yo □ Yo real continuous function, ah.

The amplification coefficient G was determined from equation (2, 5). Here, lnl Es (0+r) /Es (z+τ)
l' = Gz6 -az----, (2,7) The SBS darkness value is evaluated in each case and 2-o#3α
−1, so the dark value limit (2, 3) is Gα−1, ≧
21 ----(2,8).

In the case of no modulation, consider the following.

where PL is the input power to the fiber and Δ is the effective cross-sectional area of the introduced mode. This coincides with (aρ''/9τ)→0 in equations (2, 1) and (2, 2).

For amplitude modulation of the following equation, a(t)-1-(1-m(t)) (1-(1-ka
)') -------(2, 10) Here, ka is the depth of modulation intensity (0<ka≦100%). We get the following equation.

G= (m+ (1-m)(1-to/i)T) 2G
ss---(2,11>Gss is obtained from equation (2,9), where PL represents the peak power input to the fiber.The average power is
(m+ (1-m) (1-ka)) PL. From equation (2, 11), G becomes minimum when ka=100%. In this case, G=Gss m-' and the series number.''
It is the laser duty factor (JVt 0.5).

In the case of phase modulation, a(Ll= 1 − (2, +2)φ(
Ll-kp (m(t)-city) where kp is the PSK shift. We obtain the following equation.

G = (1-2 seedlings (1-stone) (1-cos kp)
) G55-=-, (2,13) For special values of phase shift kp, G approaches zero. For example, if...-2, Gbaselo, i.e. kp=
(2n+1)rc:n=0.1.2-. We believe that for this value of m and the above value of kp, SBS can be suppressed and more power can be input into the fiber.

We now consider the following equation for frequency modulation (FSK).

a (tl-1 where k4/2π is the keying frequency shift (l
lz), which is at least as large as bits/hour in practical use. Here, we carefully determined the Fourier frequency for the time average value of φ11+=0. This is because the corresponding frequency is the case of the maximum SBS gain. This is because the equation (2,+4) is different from the equations for 8SK and PSK because the modulation variables are related to the past history of the data stream and the encoding method. If p(ψ)d
If ψ is the probability at any special time, then the phase angle φ(
tl is in the range of ψ and ψ+dψ (-π<4≦π).

The value is obtained using the equation (2, 14), and the fourth order is suggested by also referring to the equation (2, 5).

G=PP″4G ss --------(
2, 15) Here, for example, m (tl is a unit square wave with repetition 2/B and does not represent the data sequence 010101-, B is Hairabino 1-loss, Suzuki-2, we have P= 5 inc. (k 4 / 2B). We generally imply in this sample that k, −0
as G → Gss and +/2B)) G → for l
It is 0. We proposed to use a sufficiently large frequency shift llf to suppress SBS, and further suggested that by using an unbalanced code instead of a balanced code for the SBS gain, the SBS gain can be suppressed low and the phase shift can be small. This is something that can be done within a range.

In more qualitative terms, we suggest that for a clear SBS to occur, the optoelectromagnetic field must generate (through electrostriction) a strong coherent acoustic wave within the decay time r'', etc. In the case of ASK, we believe that the optical pulse is effective in generating a Kohlen!-acoustic wave.However, in the case of PSK, if an appropriate phase shift is chosen, we believe that the purely acoustic wave produced by the optical field is The excitation that occurs during the period of binary zero works inversely to the optical field of the phase-shifted binary l (...=2, which is completely opposite to the phase of the optical field) and disappears.Similarly, the excitation of F S K case,
If the frequency shift is large enough, a continuous phase change of the optical field will occur and a small purely acoustic excitation will suppress the SBS.

We propose the following. Although recently observed low-loss silica fibers using continuous wave lasers have extremely low values for SBS, coherent optical transmission systems are designed without limitations on power level or repeater spacing due to SBS. This means that it should be done.

- In force, ASK, the dark values of SBS differ by a small numerical factor compared to continuous waves. In PSK and FSK systems with appropriate operating parameters,
Since SBS can be suppressed, there are no longer any major restrictions.

Now we know that the radar field is Δ at two optical frequencies.
ν. Let us consider the case where the equal amplitude 'A E o is increased by δ11. It is assumed that the width of each spectral line is smaller than the Prillouin scattered line width ΔνB at room temperature. The Fourier amplitude of the total input laser field is therefore a function of time t and can be expressed as:

IEL (t+) −Eo cos (ycΔ'mt
4θ)---(3, l) Here, θ is a constant. When this is substituted into the combined wave equation of the electric field of the fiber and the density wave, the result is an expression for the SBS gain coefficient.

where Gss is the field amplitude E for a single frequency laser
This is the gain coefficient of SBS that occurs when o.

One assumption in this dIW is Δshi, TI) αV, where ■ is the optical group velocity and α is the power absorption coefficient. Note that α-1 is the effective interaction length of SBS. Our suggestion is that Be 1 - Frequency Δ
When ν is much larger than Δν8, the gain G becomes small. Since the SBS threshold is inversely proportional to C<RGSmith (described above), this darkness value gradually increases. We think as follows. This appears to be because the Fourier amplitude of the laser field in the fiber is below the phase change π equal to a fraction of the beat frequency. In order to significantly generate SO3, it is necessary to generate a coherent wave with a strong Rady field within the phase reduction time Δν1 by electrostriction. However, if optical phase reversals occur more frequently than spontaneous acoustic phase changes, the acoustic waves cannot have large amplitudes, and S
BS gain becomes smaller.

61 m (Δν
The contradiction when B arises from Δshi, l+)α■. ) The spontaneous Prillouin line width of the silica fiber is 3 at λ = 1.0 pm.
8.4MlI2 and varies with λ-2 (mouth, ll
etn-man et al+ 'Br1llouin
scattering measurements
n optical glasses'+Phys,
Rev, +19791B19 + 6583-6592
(see page).

Therefore, 23 and 16M at λ-1,3 and 1.5571m
It is 1+2. From this, we believe that the beat frequency Δν would need to be at least several tens of MlI2 in order to actually achieve sufficient SBS suppression. 1 dB/k
For fibers with losses equal to or less than rn, α is 2
10'm-1, so the previous assumptions Δν, )α■ are well verified.

However, suppression of SBS may be difficult for very large Δν. From this point on, we note the following. that the difference in phonon frequency for the two laser frequencies is much less than Δνθ, allowing complete decoupling;
So---~---(3,3) where Va is the acoustic velocity and λ is the optical wavelength. Insert the following representative data for silica fibers.

It is necessary that n = 1.5 Va = 6 xlO3m, s' Δνe = 16 Mllz dn/dλ = 0.02 61m<<270 GHz, and ΔI/ is less than several 10 G11z.

There are other factors that are in principle less effective in suppressing SBS for very large Δν□. 2 if the group delay of the two frequencies is smaller than the minimum pulse interval.
It is conceivable that pulse overramping (and thus mode beating) for two frequencies may occur over the length of the fiber. However, for very large Δν1, this cannot be obtained in principle due to diffusion. But this is actually important,
isn't it. When a 300 km long silica fiber is operated at a wavelength of 1.55 μm, the group velocity dispersion is 20
ps/nm/km. If beat frequency Δν
When 0 is IG11z, the calculated difference in group delay between the two frequencies is only 50 ps.

Even in a long fiber of the type defined in the previous example 4, the threshold value of SB'S is Δshi, ll=l 15W at G11z
The remainder is considered standard.

The assumption inherent in our above analysis is that the unmodulated Radey linewidth is small compared to r. This encompasses the interest of coherent optical transmission. We suggest that SBS generated from non-uniformly extended sources requires statistical treatment of induced light scattering.

The following d1 calculation example is described as a theoretical model of the present invention.

[Example 1] Silica fiber has the following characteristics (all numbers are approximate values).

Optical absorption coefficient α at 1.3/Zm = (L, L x 10') -'m-' where α
is defined as 1/z −In (Po /Pz), and P
o and Pz are the optical powers at distances 0 and 2 along the fiber in the transmission direction in the absence of nonlinear effects.

Density ρo = 2.21X IP kgm-3 Acoustic velocity in the core Va = 6. OxlO' m sec -'Reflection coefficient of core = 1.47 Introduced mode area Δ for 1.3 μm -1,41K 10-”m 2 where A=P/Imax, where Imax is free of nonlinear effects It is the intensity of the light that falls at the center of the core with respect to the optical power P in the case.

The elasticity/light coefficient px2 in the longitudinal direction is determined by Jay Zabrial (
, J 5aprja+) definition number ( ” Aco
usto-Optics'', CI+apter vl
Wiley, 1979).

1.3. SBS line width at +1m is 1” = 7
．． l X 107 rad s-' where I' is the formula (2,
2) +J is a constant.

(1' governs the speed at which the density wave attenuates without receiving any external stimulus according to (aρ/θτ)−−r”ρ′. In Equation (2), E(−〇) ) This is for example W, Kaiser and M Mai
er, ” Stimulated-ed Raylei
gh, Br1lloutn and Raman 5p
ec-troscOpY”, La5er Hand
book Volutne 2+ed, F I Arre
cchi and E O5chulz-Duboi
s, North-11o11an(1,^m5te
R-dam, 1972, pages 1077-1150.

A PSK optical signal was input into 39 km of this fiber. The linewidth of the unmodulated light source is less than IMIIZ, and the power input into the fiber is 100 mW. The transmission of the flow of the Hynalydake is a modulation function m (tl) that takes values of 0 and 1, but this means that even if the average value m of the modulation function is 2 even if the excess time is shorter than r The ratio of transmission pinot 1. is If;Bit/
A keying phase Kp of 180° in seconds is used. A slight stimulated Brillouin scattering was observed.

[Example 2] In this example, the light source power, line width, fiber characteristics and length are the same as in Example 1. However, the binary data stream is transmitted in FSK, such that the overage time approaches 2 even though it is less than 1'-1. The transmission bit rate is 100
MBit/s and frequency shift I < 1 is 8Gtl
z was used. Very little stimulated Brillouin scattering was observed.

Example 3 Example 2 is repeated with a bit rate of IGBit/sec and a frequency shift of 1-35 GIlz. In addition, a very small amount of stimulated Brillouin scattering was observed.

[Example 4] In this example, the silica fiber used was...
The loss is 0.3 dB/km at a wavelength of 1.3 trm, and the fiber characteristics, for example α, are the same as in Example 1. The optical carrier wave is modulated using the ASKji method, and the bit ratio is 140.
MBit/sec. When the light source is on, the power is 2
I W, I G11z differ between the frequencies, and the line width is 1
It was smaller than M1lz, and m was 2. Both image frequencies are close to the wavelength of 1.3 μm. No stimulated Freyuan scattering was found.

When the reception sensitivity is -GOdBm (with an error rate of 10)
Y Yamamoto, Receiver
r'erformance evaluation o
f various digital optical
modulation-demodulaLions
ystims in Lbe 0.5-10μm wa
ve-1er+gth region”, IEE
[E, 1. Quant, EIec, +1980+Q
See page E-16, 12514259! (@) Data transmission will be possible over 300 km without repeaters.

(Explanation of Effects) As explained above, according to the present invention, an optical transmission method that is free from the adverse effects of stimulated Brillouin scattering in optical transmission using a dielectric waveguide can be obtained, and an optical transmission method using a dielectric waveguide can be obtained. stimulated Brillouin scattering can be effectively suppressed.

Further, according to the present invention, an optical transmission system can be obtained in which stimulated Brillouin scattering is suppressed by optical transmission using a dielectric waveguide.

According to the present invention, stimulated Brillouin scattering does not occur even when transmitting signals with high optical power and a narrow optical line width of the optical signal spectrum through an optical fiber or other dielectric waveguide. It has the advantage of being able to transmit optical signals with the width of an optical line, increasing the signal-to-noise ratio, and increasing the nine-body spacing in the optical transmission path.

In particular, the method of suppressing stimulated Brillouin scattering by changing the phase angle of +* transmission as the information signal is modulated is an extremely advantageous method in that it does not require any special equipment.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of the experimental apparatus. FIG. 2 is a diagram relating to Experiment 1 and showing the relationship between the optical power of the input light and the output light of the optical fiber. FIG. 3 is a diagram showing an oscilloscope 2-p display showing the relationship between the input light of the optical fiber and the scattered light in terms of wavelength, related to Experiment 1. FIG. 4 is a diagram relating to Experiment 3 and showing the relationship between the optical fiber input light and the output light of the optical fiber. If the radar operates on a single frequency. FIG. 5 is a diagram relating to Experiment 3 and showing the relationship between the optical power of the input light and the output light of the optical fiber. If the radar operates in dual frequency. Patent Applicant Free Telecommunications Company Patent Attorney Naotaka IdeL S

Claims (1)

[Claims] (1) A method of injecting one or more high-power optical signals with a narrow beam width from one end of an optical fiber, and a method of injecting the optical signals with high optical power so as to sufficiently suppress stimulated Brillouin scattering caused by the optical signals. an optical transmission method comprising: a method of changing a phase angle of a phase angle over time; (2) The optical signal is a carrier wave modulated by an IIV information signal, and the method of changing the phase angle over time includes a method of modulating the phase angle of the 1M transmission wave by an information signal. Optical transmission method described in section. (3) The information signal is a binary digital signal, and its phase angle modulation method is phase shift keying (PSK), and the modulation depth is (2n+l) π (where n is 0 or an integer). An optical transmission method according to claim (2). (4) The optical transmission method according to claim (2), wherein the optical signal is configured to equivalently change the phase of the optical signal by adding a plurality of light waves having different frequencies to the optical signal. (5) An optical signal consisting of multiple light waves with different frequencies is 1
+[li! Or the optical transmission method according to claim (4), which is generated from a plurality of light sources. (6) The optical transmission method according to claim (5), wherein each of the plurality of light sources is a radar, and each light source has a different longitudinal mode. (7) The optical transmission method according to any one of claims (4) to (6), wherein modulation corresponding to information is applied by controlling a light source. (8) The optical transmission method according to any one of claims (4) to (7), wherein the modulation corresponding to information is amplitude modulation. (9) An optical signal transmission method that performs the transmission according to any one of claims (2) to (8), and coherently demodulates the signal appearing from the optical fiber on the receiving side. (10) The optical fiber is a warped fiber, and its signal loss per month is 0°5 dB/km or more at the used wavelength, and the total length is ]Okm or more. The optical transmission method according to any of paragraph (9). (11) A method for transmitting a high-speed digital signal to an optical fiber, using one or more carrier waves with a narrow beam width and high optical power, and transmitting this carrier wave to the optical fiber using the high-speed digital signal. A digital signal transmission method characterized by applying frequency modulation to an extent that sufficiently suppresses stimulated Brillouin scattering. (12) Frequency modulation or binary frequency transition modulation (FS
K) The digital signal transmission method according to claim (11). (13) A digital signal according to claim (12) having a bit rate of 100 Mbit/S or more; (14) an optical signal carrier wave subjected to frequency modulation or phase modulation corresponding to the information G; In an information transmission method in which a high-power phase reference light wave generated from a light source with a narrow beam width as described above is transmitted from one end of an optical fiber, the phase angle of the phase reference light wave described in 2 is such that stimulated Brillouin scattering is sufficiently suppressed. An information transmission method characterized by changes in degree over time. (15) The information transmission method according to claim (14), wherein the phase reference light wave includes light wave components having a plurality of different frequencies. (+6) In an optical transmission device equipped with a light source that generates high-power output light with a narrow beam width of one or more, and an optical fiber into which the output light of this light source is input at one end, the phase of the output light of the light source is An optical transmission device characterized in that the angle is configured to change over time to an extent that stimulated Brillouin scattering is sufficiently suppressed. (I7) The optical transmission device according to claim (16), wherein the light source is configured such that its output light includes a plurality of components of different frequencies and the phase angle changes equivalently. (18) A light source that generates high-power output light with one or more narrow beam widths, means for modulating the output light of the light source in accordance with an information signal, and the output light of the light source modulated by the means. An optical transmission device comprising an optical fiber that enters from one end, characterized in that the phase angle of the output light of the light source described in (1) changes over time to an extent that sufficiently suppresses stimulated Brillouin scattering. Optical transmission equipment. (I9) The information signal is a high-speed pit-ray digital signal, the means for modulating it is a phase shift modulation (PSK) means, and the depth of the modulation is (2n+1)π. The optical transmission device according to item (18). However, n is O or an integer. (20) The information signal is a high bit rate digital signal, the modulation means is frequency transition modulation (FS) means, and the modulation depth is (2n + 1)π. The optical transmission device according to item (18). However, n is 0 or an integer. (21) The optical transmission device according to claim (18), wherein the light source is configured to generate light waves having a plurality of different frequencies. 22) The optical transmission device according to claim 21, wherein the light source is constituted by a plurality of light sources each generating light waves of different frequencies. (23) The optical transmission device according to claim (22), wherein each of the plurality of light sources is a laser, and each of the lasers operates in a different longitudinal mode. (24) The optical transmission device according to any one of claims (21) to (23), wherein the modulating means is configured to control a light source. (25) The optical transmission device according to any one of claims (21) to (24), wherein the means for applying modulation is an amplitude modulation means. (26) The optical transmission device according to any one of claims (18) to (25), wherein the receiving device includes means for coherently demodulating two optical signals appearing from the other end of the optical fiber. (27) The optical fiber is a silica fiber, the signal loss per month is 0.5 dB/km or less at the used wavelength, and the total length is IO km or more.Claim No. (18)
The optical transmission device according to any one of Items (26) to (26). (28) In an optical transmission method in which an optical signal with a high optical power and one or more narrow beam widths is transmitted through a dielectric optical waveguide, the phase angle of the optical signal is adjusted over time to the extent that stimulated Brillouin scattering is sufficiently suppressed. An optical transmission method characterized by changing with time. (29) In an optical transmission device equipped with a light source that generates one or more optical signals with a narrow beam width and high optical power, and a guided optical waveguide into which the output light of the light source is input from one end, the optical signal described in ``1. An optical transmission device characterized in that the phase angle of changes with time to such an extent that stimulated Brillouin scattering is suppressed to 1 minute.